Photodegradation of rhodamine B in aqueous solution ... - AkzoNobel

20 P. Wilhelm, D. Stephan / Journal of Photochemistry and Photobiology A: Chemistry 185 (2007) 19–25
the surface of the catalyst, the SiO2@TiO2 core–shell system is
cheaper than pure titania as the inert silica core is well available
as industrial by-product.
Rhodamine B is used as model organic dye, as it is the most
important xanthene dye and dye pollutants from the textile industry
are an important factor in environmental pollution and its
degradation mechanism has been studied quite well [24–26].
The photodegradation of rhodamine B in aqueous solution via
coated spheres was investigated by UV–vis spectroscopy and
determination of the total organic carbon (TOC).
2. Experimental
2.1. Preparation of SiO2@TiO2
SiO2@TiO2 [19,27,28] was synthesized through heterocoagulation
of silica spheres and titania nanosol. The silica particles
were synthesized according to the method described by Stöber et
al. [20]. To obtain particles in different sizes, the concentration
of the used reagents was varied. Titania sol was prepared by a
hydrolysis–condensation reaction of tetrapropylorthotitanate in
ethanol as solvent. To peptize the intermediate gel, hydrochloric
acid was added to the solution [21]. The coating was performed
by gradual addition of titania sol to silica particles dispersed
in ultra-pure water. The coating progress was monitored by
zeta potential measurements. After complete coating of the silica
particles, excess titania was removed through centrifugation
(10,000 g, 10 min) and discarding the supernatant. The coated
particles were then redispersed in ultra-pure deionized water.
More details about the preparation are given elsewhere [19].
2.2. Characterization of SiO2@TiO2
The size of titania, silica and coated silica was measured
using acoustic attenuation spectroscopy. The zeta potential of
the particles was calculated from the colloidal vibration current
(CVI) determined by electroacoustic measurements [29]. Both
methods were performed by using a DT 1200 from Dispersion
Technology. The phase composition of titania nano-particles
was determined using Rietveld refining techniques with the fundamental
parameter approach (Bruker AXS, Topas 2.1). The
morphologies of silica and SiO2@TiO2 were investigated by
scanning electron microscopy studies (SEM) (Philips XL 30
ESEM FEG).
2.3. Photodegradation of rhodamine B
The photodegradation of rhodamine B (RB) was investigated
using a Suntest CPS+ from Atlas Material Testing Solutions
as source for artificial sunlight. The applied radiation intensity
for the experiments was 550 W/m 2 , which is equivalent to the
solar intensity at high noon in the equatorial region. The test
was performed with 100 mL of water containing 10 −5 mol/L
RB and 1.5·10 −3 wt% titania and dispersed SiO2@TiO2, respectively.
The mixture was irradiated at the described intensity and
the progress of the degradation of rhodamine B was monitored
every hour by UV–vis spectroscopy (Varian Cary 50). In order
to quantify the concentration of rhodamine B, the UV–vis spectrometer
was calibrated to a concentration range between 10 −7
and 10 −5 mol/L at a wavelength of 554 nm, which corresponds
to the absorption maximum of RB. As a reference measurement
100 mL of water with 10 −5 mol/L RB was also irradiated and
monitored by UV–vis spectroscopy to investigate if degradation
also occurs in the absence of the photocatalyst.
Additionally, TOC in the mixture was determined by using a
highTOC II from Elementar Analysensysteme in order to investigate
if the dye is only photobleached or completely degraded.
For these investigations, a solution of 75 mL 4·10 −5 mol/L RB
plus 75 mL titania sol was used. This solution was also irradiated
at the described conditions except for the fact that the UV
filter was removed from the Suntest CPS+ in order to accelerate
the degradation reaction as UV rays up to a wavelength of
250 nm are applied to the system. Samples at the beginning, in
the middle and at the end of the experiment were taken for TOC
analysis.
3. Results and discussion
3.1. Characteristics of SiO2@TiO2
The size of the titania nanosol was 10 nm according to acoustic
attenuation measurements. Its isoelectric point (IEP) determined
by zeta potential measurements is at pH 6.7 (Fig. 1).
Besides that X-ray diffraction (XRD) studies were made to determine
the phase composition via Rietveld quantification. These
studies revealed anatase as major phase and rutile as minor
phase. Exact quantification was not possible due to the lack of
sharp reflection signals in the diffraction pattern, as consequence
of the nano-crystalline character of titania.
Silica spheres with a diameter of 220, 470 and 590 nm according
to acoustic attenuation spectroscopy were synthesized. Zeta
potential measurements showed that in the investigated pH range
(3.0–11.0) the zeta potential is negative. The IEP for the particles
is at pH 1.0–2.0 depending on the particle size. Fig. 1 shows the
zeta potential versus the pH value for 220 nm sized silica particles.
In order to investigate the morphology of the synthesized
Fig. 1. Change in zeta potential with the pH of silica (220 nm) (a), titania nanosol
(10 nm) (b) and SiO2@TiO2 (470 nm) (c).